The present invention relates to a headlamp capable of emitting a beam of light with a cut-off in order to form a dipped headlamp of the type known as the "standard European beam" or a fog lamp for an automobile. Thus the term "headlamp" as used herein includes those lamps known as "fog lamps".
It has been proposed to reduce the bulk of such headlamps while maintaining an equal emission of flux by employing a construction incorporating an ellipsoidal reflector having two focii, a light source at one focus and a lens beyond the other focus so that the focus of the lens coincides with the other reflector focus. A masking screen is located at the common focus to produce a cut-off beam.
Such a construction is described for example by French Pat. No. 82.20200 in the name of the present applicants.
In order to obtain a good photometric performance with such headlamps it is necessary to use a lens having a large aperture, that is to say a considerable ratio of its transverse dimensions with respect to the optical axis to its focal length, in order to pick up all the flux emitted by the reflector. However, this aperture involves significant chromatic aberrations.
These chromatic aberrations result from a difference in deflection in the plane of incidence of the different elementary colours constituting one single ray which has just struck the lens, and tend to be more significant as the light rays are deflected more, that is to say those rays deflected by the peripheral zones of the lens. These aberrations can be corrected in the same way as in photographic optics, by substituting the convergent lens by a group of juxtaposed lenses in which the respective chromatic aberrations balance each other. However, this solution is complex and costly, and an object of the present invention is to propose a similar solution, specifically adapted to the particular context of headlamps with a cut-off beam for automobiles.
Bearing in mind that the orientation of the cut-off in the case of a fog lamp for automobiles is horizontal or additionally slightly inclined with respect to the horizontal for a headlamp, these aberrations tend not to be troublesome when they are caused by the lateral zones of the lens, i.e. the zones which do not cause deflection at too great an inclination with respect to the horizontal. On the other hand, the upper and lower parts of the lens tend to cause iridescence which result in unacceptable imprecision and colouring at the cut-off of the beam.